Samsung KS57C2308 manual Reducing Instruction Redundancy, Bitr EMB

KS57C2308/P2308/C2316/P2316 SAM47 INSTRUCTION SET

Reducing Instruction Redundancy

When redundant instructions such as LD A,#im and LD EA,#imm are used consecutively in a program sequence, only the first instruction is executed, but the following redundant instructions are ignored, that is, they are handled like a NOP instruction. When LD HL,#imm instructions are used consecutively, the following redundant instructions are also ignored.

In the following example, only the “LD A, im”# instruction will be executed. The 8-bit load instruction which follows it is interpreted as redundant and is ignored:

LD	A,#im	;	Load 4-bit immediate data (#im) to accumulator
LD	EA,#imm	;	Load 8-bit immediate data (#imm) to extended accumulator

In this example, the statements “LD A,#2H” and “LD A,#3H” are ignored:

BITR	EMB
LD	A,#1H	;	Execute instruction
LD	A,#2H	;	Ignore, redundant instruction
LD	A,#3H	;	Ignore, redundant instruction
LD	23H,A	;	Execute instruction, 023H ← #1H

If consecutive LD HL, #imm instructions (load 8-bit immediate data to the 8-bit memory pointer pair, HL) are detected, only the first LD is executed and the LDs which immediately follow are ignored. For example,

LD	HL,#10H	;	HL ← 10H
LD	HL,#20H	;	Ignore, redundant instruction
LD	A,#3H	;	A ← 3H
LD	EA,#35H	;	Ignore, redundant instruction
LD	@HL,A	;	(10H) ← 3H

If an instruction reference with a REF instruction has a redundancy effect, the following conditions apply:

—If the instruction preceding the REF has a redundancy effect, this effect is cancelled and the referenced instruction is not skipped.

—If the instruction following the REF has a redundancy effect, the instruction following the REF is skipped.

+PROGRAMMING TIP — Example of the Instruction Redundancy Effect

	ORG	0020H
ABC	LD	EA,#30H	; Stored in REF instruction reference area
	ORG	0080H
	•
	•
	•
	LD	EA,#40H	; Redundancy effect is encountered
	REF	ABC	; No skip (EA ← #30H)
	•
	•
	•
	REF	ABC	;	EA ← #30H
	LD	EA,#50H	;	Skip

5-3